WASHINGTON (AP) – Two Democratic congressmen from the Midwest want President Obama and Admiral Thad Allen to order a full evaluation of the impact of the BP oil spill on Mississippi River shipping lanes.

U.S. Reps. Bruce Braley of Iowa and Phil Hare of Illinois sent a letter to Obama and Allen on Thursday urging such an evaluation.

Braley and Hare say the oil is drifting closer to the Southwest Passage, which is a critical shipping lane for farmers who rely on barge traffic to ship their crops overseas. They say they are concerned about the impact a slowdown in Mississippi River traffic could have on prices.

The two congressmen noted that a sharp decline in the prices of agricultural commodities came after Hurricane Katrina in 2005 because of a traffic slowdown on the Mississippi.